Method and apparatus for deforming a metal workpiece for upsetting rivets and for blind riveting

ABSTRACT

Method and apparatus for deforming a metal workpiece, for upsetting rivets, and for blind riveting which is accomplished by passing a stress wave through the metal workpiece or rivet sufficient to render it momentarily plastic and simultaneously forming it. The apparatus is basically comprised of means for generating a stress wave and a stress wave focussing device or amplifier. In operation, a workpiece is placed in a die or shaping means. The stress wave amplifier is then arranged to direct or focus the stress wave to the metal workpiece. Next, a stress wave is generated and passed through the stress wave focussing device, imparting a momentary state of plasticity to the metal workpiece. The momentarily plastic metal workpiece is then formed or shaped as desired.

Uiliififi States atent n91 Leitheris 51 May 8, 1973 [75] Inventor: BasilP. Leftheris, Northport, NY.

[73] Assignee: Grumman Aerospace Corporation,

Bethpage, Long Island, N.Y.

[22] Filed: Mar. 6, 1972 ['21] Appl. N0.: 231,952

Related US. Application Data [60] Division ofScr. No. 87,483, Nov. 6,1970, Pat. No. 3,646,791, Continuation-impart of Ser. No. 863,045,

Oct. 2, 1969.

[52] 1.1.5. Cl ..29/509 [51] Int. Cl ..B2ld 39/00, B2lj 15/36 [58] Fieldof Search ..72/56, 393; 29/243.52, 243.53, 243.54, 470.1, 509, 511, 509R [56] References Cited UNITED STATES PATENTS 2,465,144 3/1949 Wyatt..29/243.52

3,453,463 7/1969 Wildi ..310/27 3,108,325 10/1963 Harvey et al. ..72/563,210,509 10/1965 Aif ..219/7.5 3,417,456 12/1968 Carlson 29/47013,475,628 10/1969 McMaster et al. ...310/8.2 3,414,965 12/1968 Stau eta1 ..29/509 Primary ExaminerRichard J. Herbst Attorney-Thomas M. Gibson[57] ABSTRACT Method and apparatus for deforming a metal workpiece, forupsetting rivets, and for blind riveting which is accomplished bypassing a stress wave through the metal workpiece or rivet sufficient torender it momentarily plastic and simultaneously forming it. Theapparatus is basically comprised of means for generating a stress waveand a stress wave focussing device or amplifier. In operation, aworkpiece is placed in a die or shaping means. The stress wave amplifieris then arranged to direct or focus the stress wave to the metalworkpiece. Next, a stress wave is generated and passed through thestress wave focussing device, imparting a momentary state of plasticityto the metal workpiece. The momentarily plastic metal workpiece is thenformed or shaped as desired.

4 Claims, 4 Drawing Figures PATENT MAY 8197 SHEET 1 UF 3 FIG. 3

METHOD AND APPARATUS FOR DEFORMING A METAL WORKPIECE FOR UPSETTINGRIVETS AND FOR BLIND RIVETING REFERENCE TO RELATED APPLICATION This is adivision, of application Ser. No. 087,483, filed Nov. 6, 1970 US. Pat.No. 3,646,791.

The present application is a continuation-in-part of my earlier filedapplication Ser. No. 863,045 filed Oct. 2, 1969.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates basically to a method and apl paratus for deforming a metalworkpiece, for upsetting rivets and for blind riveting which isaccomplished by passing a stress wave through the workpiece sufficientto render the metal momentarily plastic and then forming or shaping saidmetal workpiece as desired. More specifically, this invention relates toa method and apparatus for converting electromagnetic energy into astress wave which is amplified,focused and directed to a metal workpieceor rivet such that the passage of the stress wave through the workpiecerenders the metal momentarily plastic and easily deformable.

2. Description of the Prior Art At the present time, the various methodsof metal forming basically employ energy in the formof an externallyapplied force or heat. An illustration of this can be found in metalforging, where an external force or impact is applied to a metalworkpiece, which may be hot or cold, in order to form it into thedesired shape. Another illustration is that where metal is heated tomelting and then poured as a fluid into a mold and allowed to cool.

The distinction between cold-working and hot-working of metal rests onthe relationship of the processing temperature to the recrystallizationtemperature of the metal. The recrystallization temperature is thattemperature at which there is a marked softening of the metal beingworked. Cold-working of a metal is the deformation of the metal at atemperature below the recrystallization point. More power is requiredfor cold-deformation since the metal is harder and less ductile than themetal of the hot-working process.

In the case of metal-forging, a great amount of energy is required to dothe work of metal deformation which requires the utilization of largeand bulky machinery. This is true whether the metal is cold-worked orhotworked although less force is required in hot-working the metal.Examples of such machinery include power presses, hydraulic presses,drop hammers, and steam results which are not wholly satisfactory. Rivetfailures are not uncommon and occur unpredictably in operation. Testsperformed on rivets upset by conventional riveting machines indicatewide inconsistencies of failure under load. This illustrates only oneinstance where the present metal working methods are inadequate andrequire improvement.

Another method of metal forming is by use of an expanding magnetic fieldas illustrated by the patent to Harvey et al, US. Pat. No. 2,976,907,issued Mar. 28. 1961. The Harvey patent discloses the use of a magneticfield established by a coil to create a pressure or force on the metalworkpiece thereby deforming it to the desired shape.

More recently, laboratory investigation and study has indicated thatmetals may be deformed when subjected to stress waves. For example, seeLarge Deformation Dynamic Plasticity At An Elastic-Plastic Interface byJ. F. Bell appearing in J. Mech. Phys. Solids, 1968, Vol. 16, p. 295. Inthis report it is shown that a state of plasticity can be established inmetal by a stress wave generated by striking two hardened metal barstogether.

It should also be noted that the present state of the art includes anapparatus which utilizes the electromagnetic repulsion of two magneticfields and the shock produced therefrom to hold a metal workpiece inposition while it is deformed by other means. Harvey et al., US. Pat.No. 3,108,325, FIG. 1, issued Oct. 29, 1963). This technique, however,does not utilize a stress wave to render the metal plastic to thusaccomplish the deformation.

lt has not heretofore been possible to form metallic objects by theutilization of a stress wave passing through the metal except as anobject of scientific study. There are no commercial methods whichutilize this concept nor are there any commercial or scientific methodswhich utilize electromagnetic forces to generate a stress wavesufficient to render metal momentarily plastic.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide a metal forming apparatus utilizing stress waves.

It is a further object of the present invention to provide a metalforming apparatus which is much more compact in size than present stateof the art machines.

It is another object of the present invention to provide a metal formingapparatus which may be used for riveting.

It is another object of the present invention to provide a metal formingapparatus which may beused for blind riveting.

It is another object of the present invention to provide an improvedblind rivet which may be used with the blind riveting apparatus of thepresent invention.

It is another object of the present invention to provide an improvedmethod of deforming metal workpieces and rivets.

For accomplishing the present invention, a device is provided which iscomprised basically of an energy source connected to a pancake coil, adisc shaped aluminum or copper driver adjacent to the coil, an amplifieror. focusing means adjacent to the aluminum driver, and a shockabsorbing mechanism positioned to the rear of the coil. A die or othershaping means is also necessary for forming the metal workpiece into thedesired shape. When the device is used for riveting or spot welding, abucking mass is necessary on one end of the rivet or metal workpieces.

The discharge from the energy source establishes a magnetic field aroundthe coil; this magnetic field in turn induces a current in the aluminumor copper driver. The induced current sets up a magnetic field aroundthe driver and the interaction of the two magnetic fields causes astress wave to be generated which is then propagated through thefocusing means and into the metal workpiece to render the metalworkpiece momentarily plastic.

The device for blind riveting is basically the same as above with theaddition of a second driver which produces an electromagnetic repulsionwhich acts on the blind rivet simultaneously with and in a directionopposite to the force of the first electromagneticrepulsion to therebyupset the rivet.

DESCRIPTION OF THE DRAWINGS The invention will be described andunderstood more readily when considered with the attached drawings, inwhich:

FIG. 1 is a schematic side elevational view of a forming device andworkpiece embodying the principles of the present invention;

FIG. 2 is a schematic side elevational view of a blind riveting deviceand blind rivet embodying the principles of the present invention;

FIG. 3 is a cross sectional view of a blind rivet of the presentinvention showing the rivet inserted through the holes of twosuperimposed sheets of metal; and

FIG. 4 is a cross sectional view of an upset blind rivet of the presentinvention showing the rivet clinching two superimposed sheets of metal.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method and apparatus of thepresent invention is particularly suitable for use in forging, riveting,blind riveting, punching and spot welding operations..ln fact, a singletool, with slight variations, may be used to accomplish each one ofthese operations. The variations necessary may involve a change in theenergy source and/or a different sized or shaped amplifier. The commonground running through each of these operations is that a stress wave istransmitted to the metal object which increases the energy densitywithin the metal to thereby transform the properties of the metal andallow the particular operation to be accomplished.

According to the method of the instant invention, a metal workpiece isdeformed at a temperature below its recrystallization temperature. Theworkpiece is subjected to a stress wave generated by the electromagneticrepulsion of two high intensity magnetic fields, which imparts aparticle velocity in the direction of propagation of the wave andsimultaneously increases the stress in the workpiece. The particlevelocity and stress in the work-piece are increased to a predeterminedlevel at which the metal of the workpiece becomes momentarily plastic.The axial momentum of the workpiece, produced by the propagation of thestress wave through the workpiece, causes a compressive deformation inthe direction of propagation and an expansion at right angles to thisdirection of propagation. This expansion is governed by Poisson's ratiowhich is the ratio between the lateral strain and the direct tensilestrain. This compression and resulting outward expansion of the metalworkpiece into a die or other confining area will produce the shapedesired.

In addition to the above method of deforming a metal workpiece, the samebasic concept along with 'the same apparatus may be used for spotwelding of metals. When spot welding is desired the two metal objectsare positioned such that the spot desired to be welded on each object isin contact. A stress wave is generated by the electromagnetic repulsionof two high intensity magnetic fields; this stress wave is focused onone of the metal objects. The point of contact of the two metallicobjects acts as an energy sink where the energy imparted to the metal bythe stress wave is converted to heat energy. The heat generated at thepoint of contact is high enough to locally melt the two metal objectsand therefore weld them together. It has been found beneficial to thiswelding operation to heat one of the metal workpieces, thereby allowingfor greater ease of operation.

In the case of blind riveting, the above described concept is utilizedalong with a unique blind rivet. The

blind rivet comprises a tubular body and a stem coaxiflally extendingtherethrough. The stem is provided at one end with a former which isdrawn into the tail of the tubular body when the rivet is upset, and atthe other end with means for engagement with the blind riveting tool.Part of the stem has a diameter which is greater than the insidediameter of the tubular body so that when drawn into the tubular body aninterference fit results between the stem and the tubular body. Thus, astress wave having the same properties as described above is applied tothe tubular body which becomes momentarily plastic. Simultaneously, apulling pressure is exerted on the stem which draws it into the softenedmetal of the tubular body. The former on the end of the stem contactsthe tail of the tubular body and forms a blind head therewith whileclinching the work. Because of the relative diameters of the stern andthe inside of I the tubular body, an interference fit is establishedtherebetween and also between the tubular body and the rivet hole.

Basically, the apparatus of the present invention, as depicted in FIGJ,is comprised of a power supply 8, a stress wave generator 6, a stresswave focusing means or amplifier l6, and means 20 for reflecting thestress wave transmitted through the workpiece 18.

The power supply 8 comprises an energy source 32 and switch means 30connected in parallel to a capacitor bank 28 which is then connected byswitch means 26 to the stress wave generator 6. The stress wavegenerator 6 comprises a pancake coil 12 and driver 14, situated in sideby side relationship. Driver 14 may be either aluminum or hardenedcopper. The stress wave generated in driver 14 passes through theamplifier or focusing means 16 and tip 24 and thence to the rivet 18.The head of the rivet 18 is in full contact with the hardened surface 22of the bucking mass or means 20 for reflecting the stress wavetransmitted through the workpiece l8.

In operation, the capacitor bank 28 is charged by energy source 32 whenswitch 30 is closed. After the capacitor bank is charged, switch 30 isopened and switch means 26 is closed whereby a high amperage currentpulse, of short duration, flows through the pancake coil 12, theduration of the current pulse being in the order of microseconds. A highintensity magnetic field is set up around the coil 12 and this fieldintersects driver 14, which acts as a one turn secondary winding of atransformer to thereby induce a current therein. The induced currentflowing through the driver 14 sets up a high intensity magnetic fieldaround the driver 14. The electromagnetic repulsion established by theinteraction of the two high intensity magnetic fields generates a stresswave in the driver 14 which is propagated through the amplifier l6 andtip 24'and thence through the rivet 18. The combined effect of thebucking mass 20 and the passage of the stress wave through the rivet 18causes the rivet 18 to deform axially and radially while simultaneouslythe mass of the amplifier 16 causes a head to be formed on the rivet 18at the common juncture with tip 24.

The device as described above has been successfully used to upsetstainless steel and titanium rivets and-has also been used to spot weldtwo metallic workpieces. The machine can be fixed in a frame with thework being fed thereto or it can be used portably and held by the user.The only structural difference required by the two uses is that in theportable machine a bolt is threaded into the center of the base of theamplifier l6 and extends through the recoil mechanism or shock ab sorberhaving a nut to hold the components in tight relationship; when used ina frame, a stud extends through the shock absorber l0 and into theamplifier 16 in order to keep the components centered.

In one illustrated embodiment, wherein the metal forming deviceaccording to the present invention was placed in a frame and used in ariveting operation, NAS (National Aeronautical Standard) stainless steelrivets were successfully upset. The capacitor bank was of a lowinductance and with the coil connected and electromagnetically coupledto the driver had an operating frequency of 20 KC/sec. A high electricalcurrent was fed to the coil by the capacitor bank. The coil included 18turns of a rectangularly shaped copper member onehalf inch by 0.080inch. The coil was potted in polyurethane compound equivalent to 60durometer rubber having ample elasticity, the dimensions of the coilbeing l 6 inches thick by 6 inches in diameter and having a one inchdiameter circular hole through the center. The driver consisted of606l-T4 aluminum, one-fourth inch thick by 6 inches diameter and havinga one inch circular hole through the center. The 606l-T4 aluminum wasused since it is of good conductivity and sufficient strength towithstand the forces produced in the operation. The amplifier orfocusing device, constructed of 4340 hardened steel, had a A by 6 inchdiameter cylindrical base section leading into a truncated cone 6 incheslong and having a 9 inch diameter top to which a l inch long cylindricalsection was attached. At the tip of the focusing device a rivet set wasattached having a first cylindrical section inch long and Va inchdiameter inserted into the focusing device and exposing a secondcylindrical section V: inch long and r6 inch diameter. The shockabsorber consisted of a series of rubber pads attached to the coil. Acentering stud of one diameter was inserted into the circular holes ofthe various components and into a one inch diameter by one inch deephole in the base of the focusing device. The rivets upset by this devicewere of a /6 inch length and V: inch, 3/16 inch, inch diame ter. Thevoltage required to upset these different size rivets was respectively 7KV, 5.2 KV, and 4 KV.

In another illustrated embodiment, which is a mere variation from thatdescribed above, a metal forming device was used by hand, rather than ina frame, for the riveting operation. The only variations from the aboveexample consisted of a change in dimensions of the components. The coildiameter was reduced to 4 inches, as was the diameter of the driver andthe base of the focusing device. All other dimensions remained the same.Also, instead of a centering stud, a threaded bolt was fitted into thebase of the focusing device and secured by a nut and washer at the endof the shock absorber.

The blind riveting apparatus of FIG. 2, which is a variation of thepresent invention, is comprised of a power supply 34, a stress wavegenerator 36, a stress wave focusing means or amplifier 38, a blindrivet assembly 40, and means 42 for exerting pressure on said rivetassembly 40 in conjunction with the stress wave generated by stress wavegenerator 36 to form a blind head on said rivet assembly and clinch thework.

Power supply 34 comprises an energy source 44 and switch means 46connected in parallel to a capacitor bank 48 which is then connected byswitch means 50 to stress wave generator 36. Stress wave generator 36comprises a pancake coil 52 and driver 54 situated in side by siderelationship. The driver 54 may be either aluminum or hardened copper.The stress wave generated in the driver 54 passes through the amplifieror focusing means 38 and is transmitted to the tubular body 56 of rivetassembly 40.

Blind rivet assembly 40, as most clearly shown in FIG. 3, is basicallycomprised of a tubular body 56 and stem 58. The tubular body 56 isprovided with head 60 at one end and with centrally located annularopening 62. Head 60 may be of any conventional form and is not limitedto the countersunk form depicted. The tubular body 56 is positioned inthe rivet hole provided in the work which comprises two overlappingmetal plates, designated 64 and 66. Stem 58 extends through opening 62and is provided at one end with former 68, and at the other end withserrated portion 70. The diameter of part of stem 58 is greater than thediameter of annular opening 62 such that when former 68 is drawn intothe tail of body 56 an interference fit is established between stem 58and body 56, and between body 56 and the wall of the rivet hole. Asdepicted, stem 58 is tapered from former 68 toward the serrated portion70. In another embodiment, rather than being tapered, stem 58 may bestraight with a stepped portion having a diameter larger than thediameter of annular opening 62. Whether stem 58 is tapered or straight,an interference fit will result when it is drawn by the blind rivetingdevice into the tubular body 56.

Means 42 for exerting pressure on rivet assembly 40 to set the rivet andclinch the work is basically com prised of a rod 72, means 74 on saidrod for engaging stem 58, and means for establishing a pressure on rod72 to set the rivet.

Rod 72 is slideably mounted in the riveting device and extends centrallythrough coil 52, driver 54, and partially through focusing means 38. Theend of rod 72 is positioned near tip 76 of focusing means 38 and isprovided with means 74 for engaging stem 58 extending through an openingin tip 76. Means 74 is comprised of spring 78 and gripping jaws 80 forgripping serrated portion 70 of stem 58. Spring 82 is provided tomaintain rod 72 in the forward position while travel limit stop 84limits the rearward movement of rod 72.

Means for establishing a pressure on rod 72 to set the rivet iscomprised of driver 86 connected to rod 72 and situated adjacent to coil52 on the side opposite driver 54. Driver 86 may also be either aluminumor hardened copper as driver 54. A mass 88, which may be steel,positioned behind driver 86 and also connected to rod 72 is provided tobalance the rearward thrust of the blind riveter. Foam filler 89 isprovided to allow rod 72, driver 86, and mass 88 to move rearwardlyduring the operation or an empty space may be provided.

In operation, capacitor bank 48 is charged by energy source 44 whenswitch 46 is closed. After the capacitor bank is charged, switch 46 isopened and switch means 50 is closed whereby a high amperage currentpulse, of short duration, flows through pancake coil 52, the duration ofthe current pulse being in the order of microseconds. A high intensitymagnetic field is set up around coil 52 and this field intersects driver54, which acts as a one turn secondary winding of a transformer, therebyinducing a current therein. The induced current flowing through driver54 sets up a high intensity magnetic field around driver 54. Theelectromagnetic repulsion established by the interaction of the two highintensity magnetic fields generates a stress wave in driver 54 which ispropagated through amplifier 38 and into the tubular body 56 of blindrivet assembly 40. The passage of this stress wave through the tubularbody 56 causes it to become momentarily plastic and easily deformable.Simultaneously with the electromagnetic repulsion between driver 54 andcoil 52 a second electromagnetic repulsion takes place between driver 86and coil 52. A current is induced in driver 86 by the high intensitymagnetic field of coil 52. This induced current sets up a high intensitymagnetic field around driver 86 which is repulsed by the field aroundcoil 52. This repulsion is transmitted through rod 72 to stem 58 andacts to draw former 68 toward tubular body 56 to squeeze the tubularbody 56 between former 68 and tip 76 of amplifier 38. Since thissqueezing action occurs when the tubular body 56 is in a plastic state ablind head 92 is formed at the tail of tubular body 56 and the work isclinched, as seen in FIG. 4. Also, since the diameter of stem 58 isgreater than the diameter of annular opening 62 an interference fitoccurs between stem 58 and the walls of opening 62 and between thetubular body 56 an dthe walls of the rivet hole in the work.

In designing the prototype riveter, it was necessary to design afocusing device or amplifier to concentrate the energy of the pressurepulse generated by the magnetic coil and directed to the workpiecesufficient to successfully upset the stainless steel or titanium rivets.

The analysis of this device was accomplished by using the momentum,continuity, and stress-strain relationships used in theuniaxialpropagation of stress v wave pulses in solid bars.

where u 8x/8t the particle velocity.

Since s=E1-:, equation 2 can be rewritten as:

where E Young's Modulus of Elasticity.

From Newtons First Law of Physics we have:

where p is the density of the material.

Mass continuity equation can be written as follows:

po n xo=p dx and for A =A,,

Substituting for d x1n equatlon 4, we have:

(A p (8x/8x,,) dx du/dt= ($(As)/8x) (3x/8x,,) dx

A p Sx/Sx du/dt= (8(As)/8x) (fix/8x a An/a x Substituting from equation5 we have:

p,, 8u/8t=5s/5x +s/A 514/8. (6)

Two forms were considered for the focusing device: (a) an exponentialshape governed by the formula:

and (b) a conical shape governed by the formula:

A=A,,(1x,,/R,,K)*

where:

A the area of the larger or left hand end of the the focusing device; na constant to be determined; R radius of the larger end of the focusingdevice; x the length of the focusing device; and

K the tangent of the angle described by the edge of the cone and thebase of the cone.

From experimental results we also have the relationship:

du/dt m 9) where m is a constant.

Combining equations 6, 3, 7 and 9, we are able to derive the followingequation for the exponentially shaped focusing device:

c VE/p the speed of sound in the material;

s, the stress at the larger end of the focusing device which is directlyrelated to the energy input at this point;

s the stress at the smaller end of the focusing device;

A the cross sectional area at the smaller end of the focusing device;and

2,, the time required for the particle velocity to reach a maximum.

Combining equations 6, 3, 8 and 9, we are able to derive the followingequation for the conically shaped focusing device:

s/s ROK/3 ct ((A /A) l)+x /3 ct +A /A 11) Taking the case where:

c= 200,000 in./sec. 20 X sec.

A,,//! 11.8 x,,= 4 in., and K 1.89.

Utilizing equation 10, we obtain a stress multiplication s/s )=4. 1.

Utilizing equation 1 l, we obtain a stress multiplication (s/s,,)= 17.

From measurements it was determined that the pressure developed at theelectromagnetic coil varied between about 5,000 and 9,000 p.s.i. andtherefore an average of 7,000 p.s.i. was used.

Using the cone, the theoretical stress at the rivet was determined to be17 X 7,000 p.s.i. or 119,000 p.s.i. The material of the rivet had ayield strength of 90,000 p.s.i. and, therefore, the theoretical stressdeveloped by the focusing device at the rivet was determined to besufficient. In the tests that followed, stainless steel rivets having aA inch diameter and inch length were successfully upset.

Using high speed photography it was found that the particle velocity atthe small end of the focusing ,device was approximately640 in./sec.Knowing this velocity and using the momentum equation, the stress at thesmall end ofthe focusing device can be calculated:

where p 0.3 lb./cu. in.

c= 20 X 10 in./sec. u=640 in./sec. g= 32.2 ft./sec.

therefore s l00,000 p.s.i. This calculated stress compares well withthe 1. A method of setting a blind rivet having a tubular body with ahead and extending through the rivet hole of the work to be riveted, astem extending through said tubular body, the diameter of part of saidstem being greater than the inside diameter of said tubular body, saidstem having a former which is drawn into the tail of said tubular body,said method comprising:

a. generating a stress wave;

b. applying the stress wave to the head of said tubular body of saidblind rivet;

c. controlling the intensity and duration of said stress wave to rendersaid tubular body temporarily plastic;and

d. exerting a pressure on said stem counter to and simultaneous with theapplication of said stress wave to said tubular body to draw said formerinto said tubular body, creating an interference between said stem andsaid tubular body and between said tubular body and the walls of saidrivet hole and for forming a blind head on the tail of said tubular bodyand clinching the working.

2. The method of setting a blind rivet as defined in claim 1 wherein thestep of generating a stress wave is by the electromagnetic repulsionsestablished by the interaction of a first magnetic field produced by acoil energized by a capacitor bank and a second magnetic field around adriver of suitable conductive material adjacent to said coil.

3. The method of setting a blind rivet as defined in claim 2 wherein thestep of exerting pressure on said stem comprises:

a. establishing a pressure by the electromagnetic repulsion of saidfirst magnetic field produced by said coil and a third magnetic fieldaround a driver of suitable conductive material adjacent to said coil;and

b. applying said pressure to said stem to draw said former into saidtubular body for creating an interference between said stem and saidtubular body and between said tubular body and the walls of Said rivethole and for forming a blind head on the tail of said tubular body andclinching the work.

4. The method of setting a blind rivet as defined in claim 3wherein'said second and third magnetic fields are induced in therespective drivers by the magnetic field produced by the coil.

1. A method of setting a blind rivet having a tubular body with a headand extending through the rivet hole of the work to be riveted, a stemextending through said tubular body, the diameter of part of said stembeing greater than the inside diameter of said tubular body, said stemhaving a former which is drawn into the tail of said tubular body, saidmethod comprising: a. generating a stress wave; b. applying the stresswave to the head of said tubular body of said blind rivet; c.controlling the intensity and duration of said stress wave to rendersaid tubular body temporarily plastic; and d. exerting a pressure onsaid stem counter to and simultaneous with the application of saidstress wave to said tubular body to draw said former into said tubularbody, creating an interference between said stem and said tubular bodyand between said tubular body and the walls of said rivet hole and forforming a blind head on the tail of said tubular body and clinching theworking.
 2. The method of setting a blind rivet as defined in claim 1wherein the step of generating a stress wave is by the electromagneticrepulsions established by the interaction of a first magnetic fieldproduced by a coil energized by a capacitor bank and a second magneticfield around a driver of suitable conductive material adjacent to saidcoil.
 3. The method of setting a blind rivet as defined in claim 2wherein the step of exerting pressure on said stem comprises: a.establishing a pressure by the electromagnetic repulsion of said firstmagnetic field produced by said coil and a third magnetic field around adriver of suitable conductive material adjacent to said coil; and b.applying said pressure to said stem to draw said former into saidtubular body for creating an interference between said stem and saidtubular body and between said tubular body and the walls of said rivethole and for forming a blind head on the tail of said tubular body andclinching the work.
 4. The method of setting a blind rivet as defined inclaim 3 wherein said second and third magnetic fields are induced in therespective drivers by the magnetic field produced by the coil.